1. Field of the Invention
The present invention relates to a synthetic resin vessel. More specifically, the present invention relates to a synthetic resin vessel composed of a sheet of a synthetic resin and having a top opening, wherein a protuberance of the form of a corrugation ridge is formed on the top face of a flange formed integrally with and along the edge portion of the opening, and a lid is heat-sealed through this protuberance to close the opening.
2. Description of the Prior Art
Vessels fabricated by vacuum forming or compressed air forming of a synthetic resin sheet, for example, cup-shaped vessels, are used for packaging and selling various foods and the like because the manufacturing cost is low. When such a vessel is to be circulated in the market after packaging of the content, it is necessary to close the opening by some means. According to the conventional method, a flange is formed integrally with the peripheral edge portion of the opening of the synthetic resin vessel; a lid composed of a synthetic resin film or a laminate of a synthetic resin film and an aluminum foil is placed on the flange; and the lid is heat-sealed to the edge portion of the opening of the vessel through the flange to close the opening.
According to this sealing method, especially when a vessel is fabricated by vacuum forming or compressed air forming, the thickness of the flange is likely to become uneven and the surface evenness is not sufficiently high. Accordingly, even if a lid is bonded to this flange by heat sealing, insufficient adhesion of the lid is readily caused, resulting in leakage of a liquid content. Furthermore, according to this sealing method, when the vessel content or a water drop adheres to the surface of the flange, insufficient bonding is caused also for this reason. Moreover, since a lid having a conventional seal structure is fused to the vessel through substantially the entire surface of the flange, a large force is necessary for opening the vessel by peeling off the lid. This is another problem of the conventional method.
As means for overcoming these difficulties, a part of the present inventors previously proposed in Japanese Utility Model Application Laid-Open No. 35376/1984 a synthetic resin vessel in which heat sealing can be positively accomplished, and the sealed lid can be easily peeled off. In this synthetic resin vessel, a continuous protuberance in the form of a corrugation ridge is formed along the peripheral part of a flange for bonding of a lid, and bonding is accomplished between the top ridge of the protuberance and the lid by heat sealing. However, even if this method is adopted, since positive sealing of the vessel and easy peeling off of the lid are contradictory to each other, it is very difficult to completely satisfy both of the requirements simultaneously.
Moreover, a vessel having a conventional structure in which a lid is heat-sealed through a protuberance formed on a flange to close an opening, especially one fabricated by vacuum forming or compressed air forming, is disadvantageous in that the seal strength is rendered uneven by the thickness unevenness in the sealed portion of the vessel. Still further, since the protuberance for sealing is formed along the edge portion of the opening, a large force is necessary at the beginning and end of the peeling operation, and the force required for opening the vessel varies greatly. Therefore, the content is often spilled during the opening operation. In addition, if it is intended to increase the sealability of the vessel, the peel strength is further increased, and the peeling operation becomes difficult. Still another problem is that, since a force must be applied in the peeling operation, the vessel is required to have a holding strength sufficient to resist this force. Accordingly, in this conventional vessel, the thickness of the vessel proper must be increased beyond a certain value, and increase of the manufacturing cost cannot be avoided.